Various techniques are known for stabilizing the soil or fill beneath pavement or other structural improvement to provide a stable, high integrity base therefor. Proving of a good, highly stabilized roadbed or other load base is essential if long life of the paved layer is to be achieved or if the load base is to possess the structural integrity needed to support the improvements. A variety of "aggregates" such as crushed rock have been utilized in forming roadbeds or other fill areas. When old pavement is cracked and base reconstruction is required, the usual procedure is to excavate and haul away the old base material, haul in new base material, and dump, spread, and compact the new base material. When practical, use of binding material for in-situ reconstruction of the existing base material can be much less expensive. In addition to more conventional fill and/or binder materials, a material called "fly ash" has been used to provide stabilized roadbeds by mixing a processed product such as fly ash with whatever soil already exists where the paved layer is to be deposited. The amount of fly ash or other filler/binder material to be mixed with the pre-existing soil depends heavily upon the characteristics of that soil. Engineering specifications typically indicate a required capability for carrying weights in pounds per square inch of the pavement. Types of roadbed material, such as clay, require stabilization to eliminate the plasticity of the material. Sandy types of roadbed foundation material require both binding and/or filler to fill air voids between particles of the road base material. Many types of binder material, such as cement, sand, lime, bentonite, and the like may be utilized. Stabilization of earth below buildings or other construction sites may require filler/stabilization material. It has been the practice to move substantial amounts of road base material, for example, a four foot layer, haul it away, and replace it with four feet of new filler material with binder and/or filler already mixed in. The replacement layer then is wet down and packed as necessary to achieve the needed density. Cementitious action between the binder and replacement material provides the needed stable road bed.
Fly ash is a material that is produced as a waste product from coal-fired plants of various kinds. Fly ash is so light and powdery that if a quantity of it is dumped on a horizontal surface, the fly ash "spreads out" to more or less uniformly cover a wide area. In other words, fly ash, like talcum powder, cannot be "piled up" effectively.
The known prior technique for mixing fly ash with pre-existing soil along a proposed roadway is to spread a thin layer (approximately three inches thick) along the roadway using a truck-mounted spreading device. A fine water spray is directed to limit the amount of fly ash "dust" that is generated. Then, a tilling device, such as a BOMAG Model MPH100 Recycler/Soil Stabilizer machine, is driven along the roadway on which the fly ash has been spread. The BOMAG machine includes a number of rotary tilling blades mounted under a trailing hood. Rotation of the tiller blades mixes the fly ash with the soil being tilled. If the roadbed soil is of a quality such that a large amount of fly ash is needed to achieve the degree of stabilization or void filling needed, it may be necessary to perform four or five operations each spreading three inch layers of fly ash and then re-tilling the soil before spreading the next layer of fly ash. The multiple passes are necessary because it is impossible to spread the fly ash in a layer more than three or four inches thick; however, ten to fifteen inches of fly ash may be needed to achieve proper stabilization wherein the mixed fly ash and soil react to cement the soil into the desired stable roadbed upon which a layer of pavement can be deposited so as to produce a durable, long life, paved roadway.
The highest known rate at which fly ash has been previously mixed with pre-existing soil using a tiller is 130 tons of fly ash per day. Four to five spreader truck loads of fly ash were spread on the surface and followed by a PULVAMIXER tilling device to achieve this rate.
One prior stabilizing machine is shown in Russian Patent No. 293,094, which discloses a rotary cutter that cuts through roadbed soil or the like. The rotary cutter is surrounded by a housing. Powderized binding material is fed through a gravity operated hopper into a stream of compressed air produced from a nozzle that blows the powderized binding material into the housing around the rotary cutter and into the vicinity of the rotor. A nozzle injects water sprayed to moisten the cementing mixture. The Russian patent does not disclose use of fly ash as the bonding agent or how the machine could be modified to effectively introduce fly ash into the tilled soil. Other references, including U.S. Pat. Nos. 2,937,581 and 3,753,620 disclose use of fly ash as a binder in soil to be stabilized to provide a roadbed, but do not disclose efficient, practical ways of introducing large amounts of the fly ash into the tilled soil.
It would be highly desirable to reduce the cost of mixing the needed amount of fly ash or other powdery binder or filler material into soil along a roadbed or other construction site and to provide a suitably stabilized roadbed or other soil base upon which a paved layer can be deposited so as to provide a durable, long life paved road, foundation, or the like. It also would be desirable to make it more practical to use fly ash as a filter/binder material in such applications, because disposing of fly ash often is a big problem.